The present invention relates to a hydrocarbon recovery method and more particularly relates to a method of in situ retorting an oil shale deposit to recover hydrocarbons therefrom wherein a heated gas stream is circulated through a rubblized oil shale zone within said deposit.
Oil shale deposits are shale formations wherein useful hydrocarbons exist in the form of "kerogen". While kerogen, which is a solid or semisolid, is for all practical purposes immobile within the shale, it is well known that liquid and gaseous hydrocarbons can be recovered by heating the oil shale. In recovering hydrocarbons from oil shale by use of heat, two basic techniques have evolved: surface retorting and in situ retorting.
Due to the problems normally encountered in surface retorting (e.g., cooling and disposal of spent shale), in situ retorting of oil shale is becoming more attractive as a possible means to recover hydrocarbons from oil shale. In certain in situ retorting operations, a retorting zone or gallery is formed within the oil shale deposit by first mining out a portion of the shale to create a cavity and then rubblizing the surrounding shale into the cavity by means of explosives or the like. The necessary heat for retorting is then applied to the rubblized shale either by in situ combustion or by circulating externally heated gas therethrough.
In processes where an externally heated retorting gas is used, it is common to use a portion of the recovered gaseous products, i.e., "off gas", as the retorting gas. As off gas is recovered from the retort zone, a portion of it is passed through surface retorting units where it is compressed and heated, and then reinjected into the retort zone. Surface retorting units of this type are comprised of gas compressors and gas furnaces. However, due to large pressure drops across the furnaces used to heat the gas to the high temperatures required, large quantities of power must be expanded to drive expensive compressors to overcome these pressure drops and those other pressure losses which occur throughout the circulation path of the retorting gas. Since presently all factors relating to economic success of shale oil recovery are critical, any savings in these large power requirements may affect the profits of an operation to the extent that the operational life of a particular retorting process is extended which would otherwise have to be abandoned before all recoverable hydrocarbons have been produced.